This invention relates generally to dryers for permeable webs and more particularly to through-air drying systems.
In many web processing methods, such as paper making, through-air dryers (TADs) are used for evaporative drying of the web after, before or instead of pressing devices. Typically a through air drying unit includes a hollow rotatable drying roll having a permeable cylindrical drum around which a wet web is partially wrapped as the web is passed through the unit. The web is typically supported on a continuous fabric as it is passed through the drying unit. Heated air passes through the permeable drum face and through the web and fabric so as to cause evaporative drying of the web. For reasons of energy efficiency, the heated air may be recovered after it has passed through the web and a substantial portion of the recovered air recirculated back through a heating device where it is reheated and passed back through the porous roll face and the web and fabric.
In most drying processes it is desirable to uniformly dry the web. In a continuous sheet drying process such as paper drying this means that the sheet is to be dried to uniform dryness across its width. However, the web as it enters the drying process typically varies in moisture across its width. It is said to have a moisture “profile”. That is, if the amount of moisture in the web were to be plotted against position across the web, the resulting graph would not be a horizontal line. The variations in the overall process which cause the moisture profile lead to variability in the final dryness of the product that should be corrected to improve efficiency, yield, and quality. Present methods to control or correct the product's moisture profile (referred to as “profiling”) involve corrections to sheet moisture before the drying process and within the drying process for some types of drying processes.
One known method used to correct the moisture profile is to change the drying rate across the width of the web. This is done by changing the amount of drying air flow to individual sections across the width of the web. While this is a successful method with some types of drying equipment, such as Yankee dryers having a solid drum, this is not possible with a through-air dryer because the airflow must be substantially constant across the width of the web to ensure proper operation. Accordingly, there is a need for a through-air drying unit which allows control of the moisture profile across the width of a web.